DNA polymerase III holoenzyme (holoenzyme), the chromosomal replicase of Escherichia coli, consists of a polymerase subunit in association with 9 accessory proteins. The beta accessory protein is a dimer and has the shape of a ring. The beta ring is placed around primed DNA by the 5- subunit gamma complex accessory protein in a reaction that requires ATP. The beta ring on DNA is tightly clamped to it by virtue of encircling the duplex and it slides freely along its surface. Beta also binds the polymerase subunit thereby tethering it down to DNA and conferring onto it the remarkable speed and processivity that distinguishes the holoenzyme as a chromosomal replicase. The aim of this grant proposal is to fully understand the novel enzymatic mechanism by which the gamma complex assembles the beta ring around DNA. How many ATP molecules are needed and what are their roles? What structural determinants of primed DNA does the gamma complex recognize to acids of beta contribute to its sliding motion and how does it contact the gamma complex and the polymerase? The beta sliding clamp and assembly function of the gamma complex will likely generalize to the yeast and human chromosomal replicase accessory proteins (PCNA protein and 5-protein RF-C complex) and may underlie the mechanisms of other processive processes such as translation and transcription.